You have your constructor in the wrong place.. try this:

#!/usr/bin/perl

use strict;
use warnings;

use Net::SSH::Perl;
   
open IN,  "<hosts.txt" or die "Can't open users.txt";

 while(<IN>) {
  chomp;
    my  $ssh = Net::SSH::Perl->new($_);
    $ssh->login("user1", "pass1");

    $ssh->cmd("passwd");
    $ssh->cmd("whatever");   
    $ssh->cmd("whatever");

}
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